getTotalLength() == Infinity due to float precision - Githubissues

rveciana / svg-path-properties

Pure Javascript alternative to path.getPointAtLength(t) and getTotalLength() functions. Works with Canvas & Node

MIT License

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getTotalLength() == Infinity due to float precision #61

Closed scien closed 1 year ago

scien commented 1 year ago

The following paths will cause getTotalLength() to return Infinity.

example 1: "Q 0 0 267 0 Q 391 0 512 0"
example 2:"Q 0 0 267 10 Q 391 10 512 10"

It ends up using getQuadraticArcLength() in bezier-functions.

It returns Infinity because term is infinity.
term is Infinity because Math.log(-0) is -Infinity
this only happens when (u + uuk) / (b + bbk) is zero or negative zero
this happens when (u + uuk) is 0, but (b + bbk) is very small but non-zero due to float precision during Math.sqrt

In example 1 ("Q 0 0 267 0 Q 391 0 512 0"), we get the following:

getQuadraticArcLength([ 267, 391, 512, 0 ], [ 0, 0, 0, 0 ], 1)
A = 36, B = -2976, C = 61504
k = -2.2737367544323206e-13
u = -40.333333333333336
b = -41.333333333333336
Math.sqrt(u * u + k) = 40.333333333333336
Math.sqrt(b * b + k) = 41.33333333333333
(u + uuk) = 0
(b + bbk) = -7.105427357601002e-15
term = Infinity